<?xml version="1.0" encoding="utf-8"?>
 <grammar xmlns="http://relaxng.org/ns/structure/1.0">
   <include href="flat.rng">
     <define name="book-element">
       <element name="book">
         <attribute name="id"/>
         <attribute name="available"/>
         <ref name="isbn-element"/>
         <ref name="title-element"/>
         <ref name="description-element"/>
         <zeroOrMore>
           <ref name="author-element"/>
         </zeroOrMore>
       </element>
     </define>
   </include>
   <define name="description-element">
     <element name="description">
       <text/>
     </element>
   </define>
 </grammar>
